1. Field of Invention
This invention relates to a process for selective hydrogenation of acetylene in an olefinic feed stream, particularly for ethylene purification. This invention also relates to a catalyst, its process of preparation and its use for the selective hydrogenation of acetylene, particularly for ethylene purification.
2. Prior Art
The manufacture of unsaturated hydrocarbons usually involves cracking various types of hydrocarbons and often produces a crude product containing hydrocarbon impurities that are more unsaturated than the desired product. These unsaturated hydrocarbon impurities are often very difficult to separate by fractionation from the desired product. A common example of this problem occurs with ethylene purification, in which acetylene is a common impurity. It is often difficult, industrially, to remove such undesirable, highly unsaturated hydrocarbons by hydrogenation without significant hydrogenation of the desired hydrocarbons. One example of this process is described in UK Pat. No. 916,056.
Two general types of gas phase selective hydrogenation processes for removing undesired, unsaturated hydrocarbons have come into use. One, known as xe2x80x9cfront-endxe2x80x9d hydrogenation, involves passing the crude gas from the initial cracking step, after removal of steam and condensible organic material, over a hydrogenation catalyst. Despite the large hydrogen content of such gas, which is very greatly in excess of the quantity of acetylenes that are present and which quantity should be sufficient to hydrogenate a substantial part of those acetylenes, substantially complete hydrogenation of acetylene with sufficient selectivity to produce olefins of polymerization quality is often a problem. The high concentration of hydrogen present in the front-end systems requires a very selective catalyst that does not substantially hydrogenate the ethylene. Overhydrogenation can lead to a thermal excursion in reactors, known as xe2x80x9crun-awayxe2x80x9d. Under xe2x80x9crun-awayxe2x80x9d conditions, high temperatures are experienced, severe loss of ethylene occurs and catalyst damage takes place. In addition, furnace upsets in the front-end reactor system can result in swings of CO concentration from moderate levels to very low levels. Existing front-end catalysts cannot tolerate these swings in CO concentration very well and often are prone to xe2x80x9crun-awayxe2x80x9d. In the front-end reactor system, the catalyst is also exposed to high space velocity operations 10,000-12,000 GHSV per bed. In the other type of gas phase selective hydrogenation, known as xe2x80x9ctail-endxe2x80x9d hydrogenation, the crude gas is fractionated and the resulting concentrated product streams are individually reacted with hydrogen in a slight excess over the quantity required for hydrogenation of the highly unsaturated acetylenes which are present. However, in tail-end use there is a greater tendency for deactivation of the catalyst, and consequently, periodic regeneration of the catalyst is necessary. Tail-end reactor systems operate at lower GHSV of 2500-5000 per bed. H2 addition can also be adjusted to maintain selectivity. However, formation of polymers is a major problem. Thermal excursion is not a problem.
A number of patents have discussed the selective hydrogenation of unsaturated hydrocarbons such as U.S. Pat. Nos. 4,126,645, 4,367,353, 4,329,530, 4,347,392 and 5,414,170.
The catalysts that are preferred for the selective hydrogenation reactions include palladium supported on an alumina substrate, as disclosed for example in U.S. Pat. Nos. 3,113,980, 4,126,645 and 4,329,530. Other gas phase palladium on alumina catalysts for the selective hydrogenation of acetylene compounds are disclosed, for example, in U.S. Pat. Nos. 5,925,799, 5,889,138, 5,648,576 and 4,126,645.
One of the problems with palladium on alumina catalysts is that under normal operating conditions not only is the acetylene hydrogenated, a substantial portion of the ethylene is also converted to ethane. In addition, these palladium on alumina catalysts often have relatively low stability due to the formation of large amounts of oligomers on the catalyst surface.
To overcome this problem, enhancers are added to the palladium which improve the catalyst properties. One common enhancer which is added to a palladium on alumina catalyst is silver. For example, conventional acetylene hydrogenation catalysts for ethylene purification comprising palladium and silver on a support material are disclosed in U.S. Pat. Nos. 4,404,124, 4,484,015, 5,488,024, 5,489,565 and 5,648,576. Specifically, U.S. Pat. No. 5,648,576 discloses a selective hydrogenation catalyst for acetylene compounds comprising from about 0.01 to 0.5 weight percent of palladium and, preferably, from about 0.001 to 0.02 percent by weight of silver. 80 percent or more of the silver is placed within a thin layer on the surface of the carrier body.
Catalysts comprising palladium, silver, an alkali metal fluoride and a support material, which are utilized for the hydrogenation of other feed stream impurities, such as dienes and diolefins, are disclosed, for example, in U.S. Pat. No. 5,489,565.
Catalysts comprising palladium and gold on a catalyst support which may be used for the hydrogenation of acetylenes and diolefins have also been suggested by U.S. Pat. Nos. 4,533,779 and 4,490,481. These patents disclose the use of a substantially greater amount of palladium than of gold, specifically 0.03 to about 1 percent by weight palladium and from 0.003 to 0.3 percent by weight gold. The ratio of the palladium to the gold is from 10:1 to about 2:1 as shown in Example 3 of both patents.
Other patents that disclose or suggest the use of palladium and gold on a carrier include U.S. Pat. No. 3,974,102 (isomerization of alpha-pinene) and U.S. Pat. No. 4,136,062 (oxidative dehydrogenation), FR 2,482,953 (U.S. Pat. No. 4,409,410 (selective hydrogenation of diolefins with silver and palladium)) and GB 802,100 (selective hydrogenation of acetylene with palladium and an element selected from the group consisting of copper, gold and silver, preferably silver).
A heterogeneous bimetallic palladium/gold catalyst for vinyl acetate production is disclosed in WO 97/44130. This catalyst is prepared by forming a first shell dispersion coating of colloidal palladium on a catalyst support surface and superimposing a second shell dispersion coating of colloidal gold metal on the first shell dispersion coating. An organometallic gold compound is employed to apply the gold dispersion on the catalyst support surface.
While conventional silver/palladium-based catalysts for the selective hydrogenation of acetylene have been useful, there are a number of problems that have been discovered from their use, including a relatively low tolerance to carbon monoxide concentration swings, lower selectivity than is desirable by the industry, and problems with high space velocity operation. Further, because the silver on these promoted catalysts reoxidizes quite easily during conventional preparation, transportation, installation and use, it is generally necessary to prereduce the silver-promoted catalyst in-situ before selective hydrogenation of the acetylene for the most efficient hydrogenation.
The catalysts of the invention are designed to address these problems and deficiencies in conventional ethylene purification catalysts.
Accordingly, it is an object of this invention to disclose a process for the selective hydrogenation of a C2 and C3 olefinic feed streams containing acetylenic impurities, particularly for ethylene purification.
It is a still further object of this invention to disclose a process for the front-end selective hydrogenation of acetylenic impurities, whereby the quantity of the desirable C2 and C3 olefins, particularly ethylene, is not substantially reduced.
It is a still further object of this invention to disclose process steps for the front end selective hydrogenation of a C2 and C3 olefinic feed stream containing acetylenic impurities, particularly for ethylene purification.
It is a still further object of the invention to disclose an improved palladium/gold catalyst for use in the selective hydrogenation of acetylenic impurities in front end ethylene purification.
It is a further object of the invention to disclose an improved palladium/gold catalyst containing precise quantities of palladium and gold at specific ratios on a catalyst support.
It is a further object of the invention to disclose a process for the production of a palladium/gold catalyst for the selective hydrogenation of acetylene, wherein the gold is prereduced in a wet prereduction process and does not require further reduction in-situ.
It is a still further object of the invention to disclose a palladium/gold selective hydrogenation catalyst for the selective hydrogenation of acetylene which exhibits enhanced selectivity, resistance to run-away, tolerance to CO concentration swings and improved performance at high gas hourly space velocity over conventional palladium and palladium/silver selective hydrogenation catalysts.
These and other objects can be obtained by the disclosed process for the preparation and use of a selective hydrogenation catalyst for use in a C2 and C3 olefinic feed stream containing acetylenic impurities particularly for ethylene purification, which is disclosed by the present invention.
The present invention is a process for the production and distribution of a catalyst for the selective hydrogenation of acetylenic impurities for ethylene purification comprising
preparing a carrier material in a suitable shape;
impregnating the carrier with a palladium-salt solution;
calcining the palladium-impregnated carrier;
impregnating the palladium-impregnated carrier with a gold-containing material;
calcining the palladium/gold impregnated carrier;
wet reducing the palladium and gold materials to their respective metallic states, wherein the quantity of the reduced palladium comprises from about 0.001 to about 0.028 weight percent, the amount of the gold comprises from about 0.18 to about 1.0 percent gold weight percent and the ratio of the gold to the palladium in the catalyst is in the range of about 6:1 to 50:1, and
without further reduction in-situ, distributing the catalyst.
The present invention further comprises a palladium/gold loaded catalyst for front-end ethylene purification prepared by the process described above.
The invention further comprises a process for the selective hydrogenation of acetylenic impurities for ethylene purification comprising passing an ethylene feed stream, which contains acetylenic impurities, over the catalyst described above without further prereduction of the catalyst in-situ.
The invention is a catalyst for the selective hydrogenation of acetylene for ethylene purification. The invention further comprises a process of hydrogenation of the acetylene for ethylene purification using the catalyst of the invention. The invention further comprises a process for the production of the catalyst that is useful for the selective hydrogenation of acetylene for ethylene purification.
The catalyst of the invention is designed primarily for the selective hydrogenation of acetylene in admixture with ethylene. This type of feed stream normally includes substantial quantities of hydrogen, methane, ethane, ethylene, small quantities of carbon monoxide and carbon dioxide, as well as various impurities, such as acetylene. The goal of the selective hydrogenation is to reduce substantially the amount of the acetylene present in the feed stream without substantially reducing the amount of ethylene that is present. If substantial hydrogenation of the ethylene occurs, thermal run-away can also occur.
The catalyst of the invention exhibits improved selectivity, resistance to run-away, tolerance to CO concentration swings and improved performances at higher gas hourly space velocities (GHSV). These improvements are obtainable even if the catalyst is not prereduced in-situ by the passage of hydrogen over the catalysts. In-situ prereduction is critical to enhanced performance of conventional silver/palladium hydrogenation catalysts. However, such in-situ prereduction is not possible with certain feed streams.
The catalyst that is useful for this selective hydrogenation process is comprised of a catalyst carrier onto which palladium and gold are impregnated. The catalyst carrier may be any relatively low surface area catalyst carrier (less than 100 m2/g), such as alumina, zinc oxide, nickel spinel, titania, magnesium oxide and cerium oxide. In a preferred embodiment, the catalyst carrier is an alpha alumina. The surface area of the catalyst carrier is preferably from about 1 to about 100 m2/g and more preferably from about 1 to about 75 m2/g. Its pore volume is preferably in the range of about 0.2 to about 0.7 cc/g. The catalyst carrier particles can be formed in any suitable size, preferably from about 2 to about 6 millimeters in diameter. The carrier materials can also be formed in any suitable shape, such as spherical, cylindrical, trilobel and the like. In a preferred embodiment the catalyst carrier is formed in a spherical shape.
The palladium can be introduced into the catalyst carrier by any conventional procedure. The presently preferred technique involves impregnating the catalyst carrier with a palladium metal source, such as metallic palladium or an aqueous solution of a palladium salt, such as palladium chloride or palladium nitrate, preferably palladium chloride. The extent of penetration of the palladium can be controlled by adjustment of the pH of the solution. In a preferred embodiment, the depth of penetration of the palladium is controlled such that approximately 90 percent of the palladium is contained within 250 microns of the surface of the catalyst carrier. Any suitable method can be used to determine palladium penetration, such as is disclosed in U.S. Pat. Nos. 4,484,015 and 4,404,124. After palladium impregnation, the impregnated catalyst composition is calcined at a temperature from about 400 to about 600 degrees C. for about one hour.
Once the palladium-impregnated catalyst composition has been calcined, that composition is further impregnated with a gold metal source, such as metallic gold or a gold salt solution, preferably gold chloride. The palladium/gold impregnated catalyst material is then calcined at a temperature from about 400 to about 600 degrees C. for about one hour.
In an alternative embodiment the gold and palladium salts can be co-impregnated and calcined.
The metals contained in the gold/palladium catalyst precursor are then reduced, preferably by wet reducing, using a suitable wet reducing medium such as sodium formate, formic acid, hydrazine, alkali metal borohydrides, formaldehyde, ascorbic acid, dextrose and other conventional wet reducing agents.
Once the catalyst material has been reduced, it is washed with deionized water to remove chlorides to a level of less than about 100 ppm. The reduced catalyst composition is then dried at about 100 to 200 degrees C.
The amount of gold present on the catalyst after drying is from about 0.18 to about 1.0 percent, preferably 0.21 to 0.5 weight percent based on the total weight of the catalyst. The amount of the palladium present after drying is from about 0.001 to about 0.028 weight percent, preferably 0.01 to about 0.02 weight percent, based on the total weight of the catalyst. The ratio of the gold to palladium on a by-weight basis is from about 6:1 to about 50:1, preferably 12:1 to about 30:1, most preferably from about 12:1 to about 20:1.
Following the final drying step, the palladium/gold containing catalyst is ready for use in a front-end ethylene hydrogenation reactor without in-situ reduction.
In use, the catalyst is placed in a reactor. By use of the catalyst of the invention, it is not necessary to prereduce the palladium/gold catalyst in-situ before hydrogenation of the acetylene. Such in-situ prereduction is preferred for conventional silver/palladium hydrogenation catalysts. Selective hydrogenation of acetylene occurs when a gas stream containing primarily hydrogen, ethylene, methane, acetylene and minor amounts of carbon monoxide is passed over the catalyst of the invention. The inlet temperature of the feed stream is raised to a level sufficient to hydrogenate the acetylene. Generally, this temperature range is about 35 degrees C. to about 100 degrees C. Any suitable reaction pressure can be used. Generally, the total pressure is in the range of about 100 to 1000 psig with the gas hourly space velocity (GHSV) in the range of about 1000 to about 14000 liters per liter of catalysts per hour. Existing palladium/silver catalysts do not perform consistently over this range of space velocities in front-end reactor systems.
By the process of this invention, enhanced reduction of acetylene to less than 1 ppm is possible with enhanced selectivity.
Regeneration of the catalyst may be accomplished by heating the catalyst in air at a temperature, preferably not in excess of 500 degrees C., to burn off any organic material, polymers or char.